"Parents hardly ever spot obesity in their children, resulting in damaging consequences for health," BBC News reports after a new study found a third of UK parents underestimated the weight of their child…

"Parents hardly ever spot obesity in their children, resulting in damaging consequences for health," BBC News reports after a new study found a third of UK parents underestimated the weight of their child.

The study asked parents for their views about whether their child was underweight, a healthy weight, overweight or obese, comparing this with objective measurements of the child's weight and height taken on the same day.

Researchers found most parents were only likely to think a child was overweight when they were at the top end of the very overweight category.

The study was large, with almost 3,000 participants, but may not be representative of all parents in the UK, as many of those asked did not participate.

The study also cannot tell us why parents are not recognising when their child is overweight, or the best and most effective way of improving this. But it does suggest that some help is likely to be needed to make sure parents know when their child is overweight.

If you are concerned your child may be overweight, it is better to act quickly. Research suggests obesity in the teenage years tends to persist into adulthood.

Where did the story come from?

This study was carried out by researchers from the London School of Hygiene and Tropical Medicine, the University of Bristol, University College London, and Imperial College London, and was funded by the National Institute for Health Research.

It was published in the peer-reviewed British Journal of General Practice. One of the researchers received funding from the National Institute for Health Research.

The UK media generally reported the findings of the study accurately. They also speculated about the causes of the discrepancy. The Telegraph and BBC News, for example, suggested that being overweight is now "the norm", making it hard for parents to tell when their children are not a healthy weight.

"Society as a whole has become so fat we have collectively lost our sense of a healthy weight," said the BBC. But while the authors of the study do discuss possible reasons, the study did not directly assess whether these explain the discrepancy.

What kind of research was this?

This was a cross-sectional study that compared parents' perceptions of their child's weight with objective measurements taken by school nurses. The researchers looked at how far the parents' assessments agreed with the objective assessments.

National figures show one-third of children in England aged 10 and 11 were overweight or very overweight in 2012-13. Overweight children have a higher chance of getting serious health problems such as type 2 diabetes in later life.

Previous studies showed only about half of parents can identify when their child is overweight. The researchers wanted to know at what point parents thought a child was overweight and what factors might affect this. The study didn't assess why people might wrongly estimate their children's weight.

What did the research involve?

Every year, children in reception class (aged 4 to 5) and year 6 (aged 10 to 11) at state schools in England have their height and weight measured. This information was used to classify the children's weight against national standards.

Researchers sent questionnaires to the parents of children from five primary care trusts in England who were being measured in 2010-11. They asked the parents to estimate whether their child was underweight, a healthy weight, overweight, or very overweight.

They then compared the results of the children's measurements with what the parents thought, and looked for factors that were linked to their likelihood of estimating the child's weight correctly.

The children's weight and height was converted into body mass index (BMI) and then compared with reference measurements taken from British children from 1978 to 1990.

These measurements are organised in order of increasing BMI and split into 100 groups, or centiles, of increasing BMI, each containing 1% of the reference measurements. This shows the distribution of BMI for children at different ages and is the standard way of categorising child weight.

Children are categorised as underweight if their BMI is at or below the 2nd centile, a healthy weight if they are between the 2nd centile and the 85th centile, overweight at or above the 85th centile, and very overweight (obese) if they are at or above the 95th centile.

Researchers took the objective category for each child and compared it with the parents' assessment. They then looked at what point parents would be likely to categorise a child as underweight or overweight.

They also looked at the children's age, sex, ethnic group, school year and the local area's levels of deprivation to see if they could identify factors associated with parents being more or less likely to underestimate or overestimate their child's weight status.

Because so few parents categorised their children as being very overweight (obese), the researchers combined the very overweight and overweight groups for some of their calculations.

What were the basic results?

Using the four categories of underweight, healthy weight, overweight, or very overweight, 68% of parents correctly categorised their child. Few parents (less than 1%) overestimated their child's weight status, but 31% underestimated it, believing them to be a healthy weight or even underweight when they were actually overweight or very overweight.

Only four parents described their child as being very overweight, although the objective measurements placed 369 children in that category. Parents only became more likely to categorise a child as overweight rather than a healthy weight once the child was at the extreme end of the spectrum: at or above the 99.7th centile of BMI for their age.

As an example, a child at the 98th centile, which is classed as very overweight according to national standards, had an 80% chance of being seen as a healthy weight by their parents, and only a 20% chance of being seen as overweight or very overweight.

There were similar findings for the underweight category, with parents only becoming more likely to categorise a child this way if they were at the extreme end of the spectrum (under the 0.8th centile), compared with under the 2nd centile national threshold.

The researchers said parents were more likely to underestimate their child's weight status if the children were black, south Asian, male, or older (in year 6 rather than reception). Families from better-off areas were less likely to underestimate their child's weight status.

How did the researchers interpret the results?

The researchers concluded there is "extreme divergence" between the parents' estimation of their child's weight status and their categorisation according to their BMI.

They say parents who are "unable to accurately classify their own child's weight" may be less likely to be "willing or motivated" to make changes at home that could help the child to reach and maintain a healthy weight.

The researchers suggest some reasons for the discrepancy between parents' estimates and the medical assessments, including fear of being judged and unwillingness to label a child as overweight, as well as "shifting perceptions of normal weight" because society as a whole has seen an increase in body weight.

They say there's a need for measures to bridge the gap between parents' perceptions of a child's weight status and the BMI categories used by medical professionals.

Conclusion

This study found parents in the UK are much less likely to think their child is overweight or very overweight than standard childhood BMI categories suggest. It also found parents of black or south Asian children, boys, and those from more deprived areas are more likely to underestimate their child's weight status.

But this research has some limitations. While it is based on a fairly big sample size (2,976 children who had completed parental questionnaires stating their estimated weight classification and objective weight measurements), only 15% of the parents contacted actually sent back the questionnaire, and not all of them answered the question about weight status.

This means we cannot be sure that these children are representative of all the children in the areas selected for the study (Redbridge, Islington, West Essex, Bath, and North East Somerset and Sandwell). Therefore, these findings may not be representative of all parents in those areas or other areas in the UK.

There is also some debate about the most appropriate ways to measure being overweight or obese. Research from 2014 suggests using the BMI method (where weight is compared to height) is less accurate with children than with adults.

Although researchers looked for factors affecting the parents' estimates, including ethnicity and measures of deprivation of the local area, they did not look at other factors that might also be related to parental perception – for example, the parents' own weight status, anything about the family diet, or the amount of exercise the children got. This limits the conclusions that can be drawn from the study.

While the authors discussed some possible reasons for the discrepancy between parents' estimates and the objective assessments, the study did not assess this directly, so we can't be sure what those reasons are. The study can't tell us why, for example, parents of boys or south Asian children are less likely to recognise that their child is overweight.

And we don't know if the problem is restricted to parents, or whether other professionals, such as teachers and nurses, would also underestimate a child's weight status. It's even possible that parents might not recognise that their own child is overweight, but would be able to spot it in other people's children.

It is a concern that parents don't recognise their children's weight problems – we know these children are at a higher risk of getting health problems in later life.

The authors note that a 2011 Cochrane review suggested parental support could be one important part of bringing about lifestyle changes at home and reducing childhood obesity.

Helping parents gain a better understanding of what a healthy weight looks like in a child could help reduce this problem and help improve children's long-term health.

If you're concerned your child may be too heavy, ask your GP to check whether they weigh more than they should for their age. The good news is that teaching them about healthy eating and regular exercise can lead to weight loss, as well as instilling healthy habits that may persist into adulthood.